Method of making silicon carbide-silicon composite having improved oxidation resistance

ABSTRACT

A Silicon carbide-silicon matrix composite having improved oxidation resistance at high temperatures in dry or water-containing environments is provided. A method is given for sealing matrix cracks in situ in melt infiltrated silicon carbide-silicon matrix composites. The composite cracks are sealed by the addition of various additives, such as boron compounds, into the melt infiltrated silicon carbide-silicon matrix.

This application is a continuation division of application Ser. No.08/781,494, filed Jan. 13, 1997 now U.S. Pat. 5,962,103 which is herebyincorporated by reference in its entirety.

The government may have certain rights to the invention under agovernment contract with the National Aeronautic and SpaceAdministration (NASA), contract number NAS3-26385.

FIELD OF THE INVENTION

This invention is related to a fiber reinforced silicon-silicon carbidematrix composite. More particularly, the invention is related to asilicon-silicon carbide matrix that is dispersed with glass formingelements that seal cracks in situ in the composite in a high temperaturewater-containing environment. The invention is also related to a moltensilicon infiltration method of making the composite and articles madefrom the composite material.

BACKGROUND OF THE INVENTION

Structural materials today need to operate at higher temperatures. Mostceramic materials have good long-term stability against creep andchemical attack at temperatures above the operating range for currentalloys. Because of ceramic's low fracture energies, however, ceramicsare subject to catastrophic failure. Even relatively small defects canstart propagation of cracks that can catastrophically propagate throughthe ceramic component. Therefore, measures for improving their fracturetoughness, i.e., toughening ceramics without sacrificing their excellentproperties, are sought after.

Fiber-reinforced ceramic composites are being considered as the nextgeneration of high temperature structural materials for advancedaircraft engines and gas turbines. They possess higher temperaturecapability and lighter weight than those of the currently usedsuperalloys. In these potential applications, fiber-reinforced ceramiccomposites are subjected to severe thermal and mechanical conditions.Although the fiber-reinforced composites are designed to be used belowtheir matrix cracking stress, accidental overstressing, either thermallyas a result of a thermal shock or mechanically during a foreign objectimpact, can hardly be avoided.

Cracks will be generated in the fiber-reinforced composite matrix whenthe composite is subjected to a higher stress than its matrix crackingstress. Such cracks will stay open even if the operating stress issubsequently reduced to a value below the matrix cracking stress,exposing coatings and/or fibers to the environment. As a result, theexistence of cracks in fiber-reinforced composite matrices will affectthe performance and durability of the composites, especially if thecracks are through the thickness of the composites.

These cracks can serve as a fast path for the transport of environmentalgaseous phases into the composite. Oxygen can diffuse very rapidlythrough even extremely small cracks in the matrix. The fibers and anycoating that may be on the fiber can oxidize by oxygen diffusing throughthe crack. Oxygen reacts with the fiber coating and eventually thefiber, causing local bonding between the fiber and matrix. Fiber failurewill initiate at this bonded location because of the resultant stressconcentration and fiber degradation. This process continues until theremaining fibers are unable to carry the load, and the composite failsat a stress appreciably less than the ultimate strength. The compositealso loses its tough behavior because of the strong bonding between thefiber and the matrix. Thus, a serious problem limiting the life ofceramic composites is the oxidation of the fiber coating followed byoxidation of the fiber at the base of the crack.

The ceramic composites of interest for engine applications have focusedon carbon-carbon composites, having a carbon matrix with carbon fibers,and silicon carbide composites, which have a silicon carbide matrix withsilicon carbide fibers, the fibers usually being coated. An importantlimitation to the use of carbonized structural materials is theirsusceptibility to oxidation in high temperature, oxidizing environments.Oxygen attacks the surface of the carbonized material and seeps into thepores of interstices that invariably are present, oxidizing the surfacesof the pores and continuously weakening the material. The oxidizingatmosphere reaching the fibers, carbon and graphite fibers, seriouslydegrades the composite structure.

An approach to overcome the oxidation of carbon-carbon composites hasbeen the use of glass-formers as oxidation inhibitors. The glass-formersare used as coatings surrounding the carbon matrix, and/or placedbetween carbon fiber plies. The patents that teach the use ofglass-formers as oxidation inhibitors in carbon-carbon composites areU.S. Pat. No. 4,795,677; U.S. Pat. No. 4,894,286; U.S. Pat. No.4,892,790; and U.S. Pat. No. 4,599,256.

In spite of the advances that have been made in carbon-carboncomposites, there is still a demand for improved ceramic composites withhigher temperature and mechanical capability. Silicon carbide andsilicon carbide-silicon matrix composites are currently of interest.These composites can be made by various methods. One method of makingsilicon carbide composites is the use of chemical vapor infiltration.Here, layers of cloth made of the fiber material are coated with boronnitride by chemical vapor infiltration. This takes about one day todeposit about 0.5 micrometers of boron nitride. The layers of cloth arethen coated with silicon carbide by chemical vapor deposition for about10 to 20 days. An approach to overcome the oxidation of silicon carbidecomposites has been the use of an oxygen-scavenging sealant-formingregion in intimate contact with the ceramic fibers and a debondinglayer, which is in intimate contact with the ceramic fibers, asdescribed in U.S. Pat. No. 5,094,901.

A method of making silicon carbide-silicon matrix composites reinforcedwith silicon carbide-containing fibers is by using molten silicon meltinfiltration into a preform. In this process, silicon carbide fibers arebundled in tows and coated with a coating or combination of coatingsselected from the group consisting of boron nitride, pyrolyzed carbon,silicon nitride, carbon, and mixtures thereof. 35 In one version ofsilicon carbide-silicon composites, the coatings comprise layers ofboron nitride and silicon carbide or silicon nitride. The tows arelaminated to make a structure, which is then infiltrated with moltensilicon. In these methods a boron nitride coating on the fiber is usedto protect the fiber from attack by molten silicon or for debonding.Another method used to make silicon carbide-silicon composites usesfibers in the form of cloth or 3-D structure, which are layered into thedesired shape. Boron nitride coating is deposited on the cloth layers bychemical vapor infiltration as mentioned above, and silicon carbidecoating is deposited also by chemical vapor infiltration for about 5days to achieve a thickness of about 2 micrometers. The structure isthen processed in a slurry and melt infiltrated with molten silicon. Themolten silicon may contain minute amounts of boron.

Recently, reinforcing silicon carbide-silicon matrix composites withstrong silicon carbide fibers have been shown to increase their fractureenergy substantially and exhibit tough failure mode. The increasedfracture toughness of silicon-silicon carbide matrix composites,combined with their high creep resistance and chemical stability at hightemperatures, make them promising materials for use as structuralcomponents in hot sections of heat engines and gas turbines.

When silicon carbide-silicon matrix composites develop fine cracks as aresult of loading beyond the first matrix cracking stress, the siliconcarbide fiber as well as the crack surface is exposed to oxidativeenvironments. This can occur in dry oxidative environments as well aswet or water vapor-containing environments, such as encountered underhumid conditions in combustion engines where there are combustiongaseous products of fuels. The oxidative attack is rapid at hightemperatures. The oxidation of the crack surface and the fiber may makethe composite brittle. The result may be a weaker composite andpremature failure of a component part made from the siliconcarbide-silicon composite.

Thus, there is a need for a ceramic composite that successfully sealscracks in silicon carbide-silicon matrix composites to prevent the readyaccess of oxygen at high temperatures, above about 600° C. There is alsoa need for a method to make silicon carbide-silicon matrix compositesand articles made from molten silicon infiltration that heal matrixcracks in dry and water vapor-containing environments at hightemperatures, greater than about 600° C.

SUMMARY OF THE INVENTION

These needs are satisfied by the development of fiber reinforced siliconcarbide-silicon matrix composite having improved oxidation resistance athigh temperatures in dry or water-containing environments. The inventionprovides a method for sealing matrix cracks in situ in siliconcarbide-silicon matrix composites. The composite cracks are sealed bythe addition of various additives, such as boron compounds, into thesilicon carbide-silicon matrix. A crucial factor to successfully sealcracks is the atomic ratio of atom to silicon atoms in the matrix. Thematrix boron additives oxidize once they are exposed on crack surfaces.These oxidation products help seal the crack before damage to theunderlying fiber and its coating takes place. The benefit is thepreservation of the composite properties when the siliconcarbide-silicon matrix composite is subjected to higher stresses thanthe matrix cracking stress.

Briefly then, one aspect of the invention is a method for in situsealing of cracks in silicon carbide-silicon matrix compositesreinforced with coated fibers, comprising the steps of: selecting apreform having fibrous material and an admixture comprising particlesselected from the group consisting of carbon particles, silicon carbideparticles, and mixtures thereof, and a boron compound selected from thegroup consisting of boron carbide (B₄C), titanium borides (such asTiB₂), zirconium borides (such as ZrB₂), aluminum borides (such as AIB₂and AIB₁₂), calcium borides (such as CaB₆), boron silicides (such asSiB₆), and mixtures thereof; infiltrating at least molten silicon intosaid preform to form a silicon carbide-silicon matrix composite havingboron compounds dispersed in said matrix, where said boron compoundssubsequently oxidize at a crack surface to provide a glass sealant insaid crack.

In another aspect of the invention, there is provided a ceramiccomposite capable of self-healing cracks, comprising coated fibers in asilicon carbide-silicon matrix containing boron selected from the groupconsisting of boron carbide, titanium boride, zirconium boride, aluminumborides, calcium boride, boron suicides, and mixtures thereof. Apreferred boron-containing compound is B₄C. It is believed that the B₄Cpartially dissolves into the silicon melt and becomes more uniformlydistributed in a finer scale throughout the matrix.

Yet another aspect of the invention includes an article of manufacturefor use in an engine, said article made of a ceramic composite having anelemental silicon phase containing dissolved boron, a silicon carbidephase, fibrous material, and boron-containing compounds dispersedthroughout the composite, where said boron-containing material isselected from the group consisting of boron carbide, titanium boride,zirconium boride, aluminum borides, calcium boride, boron silicides, andmixtures thereof.

One object of the invention is to provide a ceramic composite withincreased oxidation resistance. The oxidation resistance is increased ina fiber reinforced silicon-silicon carbide matrix composite in thepresence of matrix cracks. Another object of the invention is to sealthe cracks in the composite matrix in situ with rapidly formingoxidation products on the crack surfaces, so as to block the path forfurther oxygen diffusion. Still another object of the invention is toprovide a method to seal cracks in water vapor-containing environments,e.g., in engine applications.

Those skilled in the art will gain a further and better understanding ofthe present invention from the detailed description set forth below,considered in conjunction with the figures accompanying and forming apart of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing the top view (1 a) and cross-section (1 b)of a Vickers indentation to simulate matrix cracks in the composites.

FIG. 2 is scanning electron micrographs of the surface of a SiC—B₄Csample after indentation at magnifications of 2,000 and 10,000,respectively.

FIG. 3 is SEM micrographs of the surface of a sintered SiC/B₄C sample(B/Si=0.25), subjected to oxidation at 900° C. in flowing 90% H₂O-10% O₂for 24 hours. Little or no crack sealings are observed.

FIG. 4 is SEM micrographs showing the surfaces of (4 a) a silicon meltinfiltrated SiC/B₄C sample (B/Si=0.18) and (4 b) a sintered SiC/B₄Csample (B/Si=0.25), both subjected to oxidation at 900° C. in flowing90% H₂O-10% O₂ for 24 hours. FIG. 4a, with lower additive content thanthe sintered sample has more oxidation product left on the surface andshows complete sealing such that the original indentation mark(diagonal) cannot be seen. On the other hand, little or no crack sealingis observed on the sintered sample and the cracks and indentation markcan be clearly seen.

FIG. 5 is a graph showing the width of a crack sealed as a function ofB/Si ratio in the silicon melt infiltrated materials oxidized in 90%H₂O-10% O₂ for 24 hours and 4 hours.

FIG. 6 is a graph showing the difference in composition between sampleswith B₄C addition by silicon melt infiltration and by sintering.

DESCRIPTION OF THE INVENTION

Silicon carbide-based composites, such as fibrous siliconcarbide-silicon matrix composites, that are processed by silicon meltinfiltration, and that have the capability of sealing matrix cracks inwater-containing oxidizing high temperature environments, are providedby this invention. The present invention produces a ceramic compositewith a porosity of less than about 20% by volume, with the capacity toheal matrix cracks in situ in a silicon carbide-silicon compositematrix, comprising a fibrous material of which the fibrous materialcomponent comprises at least about 5% by volume of the composite; and acomposite matrix having at least about 1% by volume of a phase ofelemental silicon comprising substantially silicon; and boron dispersedthroughout the matrix where an atomic ratio of boron to silicon atoms inthe composite matrix is between about 0.05 to about 0.40, and the boronis present in the form of elemental boron or at least oneboron-containing compound. The elemental silicon phase comprisessubstantially silicon, but may have other dissolved elements, such asboron. It has further been discovered that matrix cracks seal in situ inhigh temperature, wet or dry environments.

Another embodiment of the invention is provided by a method of making asilicon-silicon carbide composite capable of healing cracks in situ inthe composite matrix, comprising the steps of: depositing at least afirst coating on a silicon carbide-containing fibrous material whichsubstantially covers an outer surface of said fibrous material; admixinga particulate material selected from the group consisting of carbon,silicon carbide, and mixtures thereof, and at least one boron-containingmaterial with said fibrous material; forming said admixture into apreform; infiltrating said preform with an infiltrant comprisingsubstantially molten silicon; and cooling said infiltrated preform toproduce the silicon-silicon carbide matrix composite, where a ratio ofboron atoms to silicon atoms in said matrix is between about 0.05 toabout 0.40.

As used herein, “carbon” includes all forms of elemental carbonincluding graphite, particles, flakes, whiskers, or fibers of amorphous,single crystal, or polycrystalline carbon, carbonized plant fibers, lampblack, finely divided coal, charcoal, and carbonized polymer fibers orfelt such as rayon, polyacrylonitrile, and polyacetylene. “Fibrousmaterial” includes fibers, filaments, strands, bundles, whiskers, cloth,felt, and a combination thereof. The fibers may be continuous ordiscontinuous. Reference to silicon carbide-containing fiber or fibrousmaterial includes presently available materials where silicon carbideenvelops a core or substrate, or where silicon carbide is a core orsubstrate. Other core materials which may be enveloped by siliconcarbide include carbon and tungsten. The fibrous material can beamorphous, crystalline, or a mixture thereof. The crystalline materialmay be single crystal or polycrystalline. Examples of siliconcarbide-containing fibrous materials are silicon carbide, Si—C—O,Si—C—O—N, Si—C—O-Metal, and Si—C—O-Metal where the Metal component canvary but frequently is titanium or zirconium. There are processes knownin the art which use organic precursors to produce siliconcarbide-containing fibers which may introduce a wide variety of elementsinto the fibers.

In carrying out the present process, a coating system is deposited onthe fibrous material which leaves at least no significant portion of thefibrous material exposed, and preferably, the entire material is coated.The coating system may contain one coating or a series of coatings. Thecoating should be continuous, free of any significant porosity andpreferably it is pore-free and significantly uniform. Examples ofcoating systems are boron nitride and silicon carbide; boron nitride,silicon nitride; boron nitride, pyrolyzed carbon, silicon nitride, etc.Examples of further coatings on the fibrous material that arecontemplated for this invention are nitrides, borides, carbides, oxides,silicides, or other similar ceramic refractory material. Representativeof ceramic carbide coatings are carbides of boron, chromium, hafnium,niobium, silicon, tantalum, titanium, vanadium, zirconium, and mixturesthereof. Representative of the ceramic nitrides useful in the presentprocess is the nitride of hafnium, niobium, silicon, tantalum, titanium,vanadium, zirconium, and mixtures thereof. Examples of ceramic boridesare the borides of hafnium, niobium, tantalum, titanium, vanadium,zirconium, and mixtures thereof. Examples of oxide coatings are oxidesof aluminum, yttrium, titanium, zirconium, beryllium, silicon, and therare earths. The thickness of the coatings may range between about 0.3to 5 micrometers.

The fibrous material may have more than one coating. An additionalprotective coating should be wettable with silicon and be about 500Angstroms to about 3 micrometers. Representative of usefulsilicon-wettable materials is elemental carbon, metal carbide, a metalcoating which later reacts with molten silicon to form a silicide, ametal nitride such as silicon nitride, and a metal silicide. Elementalcarbon is preferred and is usually deposited on the underlying coatingin the form of pyrolytic carbon. Generally, the metal carbide is acarbide of silicon, tantalum, titanium, or tungsten. Generally, themetal silicide is a silicide of chromium, molybdenum, tantalum,titanium, tungsten, and zirconium. The metal which later reacts withmolten silicon to form a silicide must have a melting point higher thanthe melting point of silicon and preferably higher than about 1450° C.Usually, the metal and silicide thereof are solid in the presentprocess. Representative of such metals is chromium, molybdenum,tantalum, titanium, and tungsten.

Known techniques can be used to deposit the coatings which generally isdeposited by chemical vapor deposition using low pressure techniques.

As stated above, the coated fibrous material is admixed with at least acarbon or silicon carbide or mixture of carbon and silicon carbidematerial and boron or at least one boron-containing compound. Otherelements or compounds may be added to the admixture to give differentcomposite properties or structure. The particular composition of theadmixture is determinable empirically and depends largely on theparticular composition desired, i.e., the particular properties desiredin the composite. However, the admixture always contains sufficientelemental carbon, or silicon carbide, or mixtures of carbon and siliconcarbide, to enable the production of the present silicon-silicon carbidecomposite. Specifically, the preform should contain sufficient elementalcarbon or silicon carbide or mixtures of carbon and silicon carbide,generally most or all of which may be provided by the admixture and someof which may be provided as a sacrificial coating on the fibrousmaterial, to react with the molten silicon infiltrant to produce thepresent composite, containing silicon carbide, silicon, and theboron-containing silicon carbide. Generally, elemental carbon rangesfrom about 5% by volume, or from about 10% or 20% by volume, to almostabout 100% by volume of the admixture.

The boron-containing compound in the admixture in the preform is presentin a sufficient amount to have an atom ratio of boron to silicon in thesilicon-silicon carbide matrix of between about 0.05 to 0.40. Thepreferred range is about 0.10 to 0.25, and the most preferred range isabout 0.11 to 0.19. It has been discovered that this range of boron tosilicon atoms present in the composite matrix will seal the cracks about1 micrometer wide in about four to twenty-four hours in a watervapor-containing environment. At four hours an atomic ratio of boron tosilicon was about 0.19. The cracks sealed completely in twenty-fourhours with an atomic ratio of boron to silicon of about 0.11. The crackssealed with these ratios of boron to silicon are about 1 micrometerwide, or 1.1 micrometers wide. Wider cracks may be sealed if the atomicration of boron to silicon is adjusted, as well as the time andtemperature.

Preferred boron-containing compounds are carbides and silicides. A mostpreferred compound is boron carbide, which is admixed with carbon orsilicon carbide or mixtures of carbon and silicon carbide in the preformprior to the step of molten silicon infiltration. Specific examples ofboron-containing compounds are boron carbide, titanium borides,zirconium borides, aluminum borides, calcium borides, boron silicides,and mixtures thereof.

The mixture of carbon or silicon carbide or carbon and silicon carbideand at least one boron-containing compound in the preform can be in theform of a powder and may have an average particle size of less thanabout 50 microns, more preferably less than about 10 microns. The moltensilicon that infiltrates the preform is comprised substantially ofsilicon, but may also contain elemental boron, which has limitedsolubility in the molten silicon. The silicon infiltrant may alsocontain boron-containing compounds or other elements or compounds.

The admixture in the preform containing the carbon or silicon carbide ormixture of silicon carbide and carbon, and boron-containing compound iswetted by the molten silicon infiltrant. In carrying out the presentprocess, the preform is contacted with the silicon infiltrant by aninfiltrating means. The infiltrating means allow the molten siliconinfiltrant to be infiltrated into the preform. U.S. Pat. No. 4,737,328,incorporated herein by reference, discloses an infiltration technique.In the present process, sufficient molten silicon infiltrant isinfiltrated into the preform to produce the present composite.Specifically, the molten silicon infiltrant is mobile and highlyreactive with any carbon present in the preform to form silicon carbide.Pockets of a silicon phase also form in the matrix. The boron-containingcompounds in the preform are incorporated into the silicon-siliconcarbide matrix during the matrix formation in the molten siliconinfiltration step.

The period of time required for infiltration is determinable empiricallyand depends largely on the size of the preform and extent ofinfiltration required. Generally, it is complete in less than about 60minutes, and often in less than about 10 minutes. The resultinginfiltrated body is cooled in an atmosphere and at a rate which has nosignificant deleterious effect on it.

The present composite then is comprised of coated fibrous material and amatrix phase. The matrix phase is distributed through the coated fibrousmaterial and generally it is substantially space filling and usually itis interconnecting. Generally, the coated fibrous material is totallyenveloped by the matrix phase. The matrix phase contains a phase mixtureof silicon carbide and silicon. The boron-containing compounds aredispersed throughout the matrix. The fibrous material comprises at leastabout 5% by volume, or at least about 10% by volume of the composite.The matrix contains a silicon carbide phase in an amount of about 5% to95% by volume, or about 10% to 80% by volume, or about 30% to 60% byvolume, or about 45% to 55% by volume, of the composite. The matrix maycontain an elemental silicon phase in an amount of about 1% to 30% byvolume of the composite. Boron is present in the matrix in an amountmeasured by the atomic ratio between boron and silicon. This can beabout 0.05 to 0.40, preferred about 0.10 to 0.25, and most preferredabout 0.11 to 0.19.

The following examples further serve to demonstrate, but not limit, theinvention.

EXAMPLES

TABLE 1 shows the experimental conditions used in the examples todemonstrate crack-sealing in silicon-silicon carbide composites. The 90%water content in the atmosphere (P_(H2O)=0.9 atm) represents an upperlimit for the water vapor pressure in engine combustor operatingconditions. Four different high temperatures were used: 600° C., 700°C., 800° C., and 900° C. and at three different times, the effects ofcrack-sealing were looked at (40 minutes, 4 hours, and 24 hours).

TABLE 1 Experimental Conditions for Crack-Sealing Study Temperature600°, 700°, 800°, and 900° C. Atmosphere Dry O₂ and 90% H₂O + 10% O₂Time 40 min, 4 hr, 24 hr

The boron-containing compounds used in the examples for sealing matrixcracks are summarized in TABLE 2. In the cases that the boron-containingcompounds were coarse in size, efforts were made to extract fineparticles from the raw supplies by centrifugal sedimentation.

TABLE 2 Boron-containing Materials Used in Crack-Sealing Study Density BContent Material (g/cm³) Particle Size (wt %) SiC 3.21 <1 μm 0 B₄C 2.52<5 μm .76 TiB₂ 4.52 99% <6 μm >28.5 ZrB₂ 6.09 99% <10 μm >18.7 SiB₆ 2.43−200 mesh ˜68 CaB₆ 2.46 −20 mesh >55 AlB₂ 3.16 2-5 μm >44 AlB₁₂ 2.58 3μm ˜83

For sintered samples, preparation of the composites were done asfollows. Silicon carbide and various boron additives were mixed with 3weight % Carbowax™ as a binder, and Novolac™ in methanol solution (10%)to supply 0.5 weight % free carbon for the sintering. For titaniumboride, zirconium boride, and calcium boride additives, 0.5 weight % ofelemental boron was also added as a densification aid. Differentboron/silicon atomic ratios were obtained by varying the amount of theadditives. The mixtures were ball milled for 6 hours in isopropanol withzircon media. The slurries were then oven-dried and sieved through a 40mesh screen. Powder compacts were prepared by cold uniaxial and coldisostatic pressing before densification. Pressureless sintering wasperformed on SiC—B₄C samples in a graphite furnace under flowing argonat 2100° C. for 20 minutes. Hot pressing was performed on samples withall other boride additives in an induction heated furnace under flowingN₂ at 2000° C. and 60.7 MPa for 30 minutes.

Densification of the silicon carbide and additive mixtures to form thecomposite was achieved by pressureless sintering, hot pressing, and meltinfiltration. Silicon melt infiltration was part of the method of makingthe inventive composite. In contrast, the sintering and hot pressingmethods were used to demonstrate the boron-containing compound effectson crack sealing in composites made by sintering or hot pressing orchemical vapor infiltration, processes in which no elemental siliconphase is present.

Silicon melt infiltration was employed for the silicon carbide/boroncarbide and silicon carbide/titanium boride mixtures.

Pure silicon was used as the infiltrant. Powder mixtures were colduniaxially pressed at 53 MPa into preforms of about 60% density. Thepreforms were placed in a boron nitride coated graphite boat and buriedin a pile of silicon powder. The infiltration was carried out in agraphite resistance furnace under vacuum at 1425° C. for 30 minutes.After densification, the samples were cut into cubes with about 3millimeters in each dimension, and one side of the sample was polishedto 3 micrometer finish with diamond pastes. Indentation cracks wereintroduced on the polished surfaces. Vickers indentation was used tointroduce orthogonal cracks in the composites as illustrated in FIG. 1aand 1 b. FIG. 1a shows the diagonal length of the indent 7, the maximumcrack opening 8, and the crack length 9. In FIG. 1b it is shown that thecracks 17 are normally of half-penny shapes 15 spreading away from theplastic zone 11 underneath the indent 13. Cracks have their maximumopening at the indent corners and the width gradually diminishes to zeroat the tip. FIG. 2 shows the scanning electron micrographs of thesurface of an as-indented SiC—B₄C sample.

Oxidative experiments were conducted in a vertical tube furnace withplatinum heating elements. Specimens were hung inside an enclosed quartztube in baskets made of platinum wire. The atmosphere in the quartz tubewas controlled. In the case of oxidation under dry environment, 100cc/min of oxygen was passed through the specimen chamber. For oxidationunder H₂O-containing environment, 0.33 ml/min of purified deionizedwater (equivalent to 450 cc/min water vapor at 25° C.) was pumped intoan evaporator and the generated water vapor was then mixed with 50cc/min oxygen to create an environment of about 90% H₂O-10% O₂. Oxidizedsamples were characterized by optical and scanning electron microscopy(SEM) to determine the extent of crack sealing, and energy dispersiveX-ray (EDX) analysis for elemental information. The mass change ofsamples before and after the oxidation were also monitored in someexperiments to evaluate the amount of oxidation product.

Example 1 Crack Sealing under Dry Oxidative Environments

Sintered samples of silicon carbide with different concentrations ofB₄C, varying from B/Si=0.015 to 0.34, were used for the study of theminimum additives in terms of B/Si atomic ratio required to achievecrack sealing in silicon carbide matrices. It was found that cracksealing occurs at a B/Si ratio as low as 0.05. To compare with the cracksealing capability of B₄C in dry oxidizing environments, a variety ofdifferent boride additions were examined. Complete crack sealing similarto that by B₄C was achieved with all boron-containing additives in dryoxidizing environments. However, because B₄C has low density and goodchemical compatibility with silicon carbide and does not introduce extraelements in the system, it is a possible choice for crack sealing in dryoxidative environments.

Silicon carbide plus B₄C or TiB₂ samples were also prepared by siliconmelt infiltration. It was demonstrated that melt infiltrated SiC/B₄C andSiC/TiB₂ materials all have crack sealing capability in the dryoxidizing environment. Further tests were done on samples of SiC/B₄C andSiC/TiB₂ to study the effects of time and temperature on crack sealingin dry oxidizing environments.

It was shown that effective sealing is obtained under practically allconditions studied with complete sealing of about 1 μm wide cracks intimes as low as 4 hours at temperatures of 800° C. and higher. At lowertemperatures of 600° C. and 700° C., the sealing was also observed butonly cracks several tenths of a micron were sealed.

Example 2 Crack Sealing under H₂O Vapor-Containing OxidativeEnvironments in Sintered Silicon Carbide Composites

Sintered SiC/B₄C samples were tested in a water vapor-containingenvironment, about 90% H₂O-10% O₂. The cracks in the sintered SiC/B₄Csamples were not sealed. FIG. 3 shows that after treatment at about 900°C. for about 24 hours under about 90% H₂O-10% O₂, sealing did not occur.Most of the cracks remained unsealed. B₄C as a boron-containing compoundin sintered silicon carbide composites does not provide an adequatecrack sealing capability in sintered SiC/B₄C materials under high steamenvironments, such as the 90% H₂O-10% O₂ environment.

Example 3 Crack Sealing Behavior of Silicon Melt InfiltratedSilicon-Silicon Carbide Composites under H₂O Vapor-Containing OxidativeEnvironments.

When samples made by silicon melt infiltration of preforms consisting ofsilicon carbide and boron carbide particles were subjected to the watervapor-containing environments, these samples exhibited superior cracksealing behavior compared to the sintered and hot pressed samples withTiB₂. FIG. 4 shows a melt infiltrated SiC/B₄C sample. The improved cracksealing capability of boron carbide in silicon melt infiltrated samplesmay be due to the fact that boron carbide is partially dissolved in thesilicon melt and distributed more uniformly in the matrix. The formationof B₂O₃ promotes the oxidation of silicon carbide as the oxygendiffusivity is much higher in B₂O₃ or SiO₂—B₂O₃ phases than in puresilicon oxide. Since in melt infiltrated samples the boron-containingcompounds are mixed with silicon and silicon carbide in a much finerscale, the SiO₂—B₂O₃ oxidation product is more uniformly distributed,providing better and continuous sealing.

FIG. 5 is a graph showing the width of a crack sealed as a function ofB/Si ratio in the silicon melt infiltrated materials oxidized in about90% H₂O-10% O₂ at about 900° C. for about 24 hours and about 4 hours.The results in FIG. 5 show that the crack width sealed increases withtime and B/Si ratio. The B/Si ratio is important, and a B/Si ratio overabout 0.11 is particularly useful at 900° C. The critical B/Si ratio maybe a function of the water vapor content, gas velocities, andtemperature. It is also possible that a higher value of the B/Si ratiomay be required at lower temperatures where the sealing will takelonger. Therefore, it is recommended that for effective crack sealing,the B/Si ratio in silicon melt infiltrated composites be over about 0.11and preferably over about 0.15.

FIG. 6 shows a comparison between silicon carbide composite samples withboron carbide, made by silicon melt infiltration (B/Si=0.18) andsintering (B/Si=0.25), oxidized in water vapor-containing environmentsfor 24 hours at different temperatures. While the sintered sample barelyshowed any crack sealing after 24 hours at 900° C., the melt infiltratedsample was able to seal cracks of greater than 1.2 micrometers width at800° C. in 24 hours. The small sealing (about 0.2 μm) shown in FIG. 6for sintered samples represents our lowest limit to predict cracksealing and may actually be crack sealing caused by high temperatureexposure rather than crack sealing by oxidation.

What is claimed:
 1. The method for in situ sealing of cracks in siliconcarbide-silicon matrix compositions containing coated fibers, carbonparticles, silicon carbide particles, or mixtures of carbon and siliconcarbide particles and at least one boron contain material, the atomicratio of boron atoms to silicon atoms in said matrix being between about0.05 and about 0.40, which comprises infiltrating a preform of saidcomposition with molten silicon.
 2. The method according to claim 1where said boron-containing material is elemental boron, a boron carbideor a boron silicide or mixtures thereof.
 3. The method according toclaim 2 where said boron-containing material is selected from the groupconsisting of boron carbide, titanium borides, zirconium borides,aluminum borides, calcium borides, boron suicides, and mixtures thereof.4. The method according to claim 3 where said boron-containing materialis boron carbide.
 5. The method according to claim 1 where said preformhas fiberous material.
 6. The method according to claim 5 where saidfibrous material has at least a first outer coating coveringsubstantially all of a surface of the fibrous material.
 7. The methodaccording to claim 6 where said first outer coating is selected from thegroup consisting of nitrides, borides, carbides, oxides, silicides andmixtures thereof.
 8. method according to claim 7 where said firstcoating comprises boron nitride.
 9. The method according to claim 8where said fibrous material also contains a coating of silicon carbideor silicon nitride.
 10. The method according to claim 6 where saidfibrous material has a second coating on said first coating.
 11. Themethod according to claim 5 where said fibrous material is a siliconcarbide-containing fiber.
 12. The method according to claim 1 where saidmolten infiltrant comprises silicon with dissolved boron.
 13. The methodaccording to claim 1 where said boron-containing material subsequentlyoxidizes at a crack surface to provide a glass sealant in said crack ina dry or water-containing environment.